A frequency resource needs to be used in wireless communication. An operator pays for a right to use a frequency to legitimately use these spectrums to deploy a wireless communications network, and another person and organization cannot use these spectrums to perform wireless communication, where a spectrum of this type is referred to as a licensed spectrum.
Besides the licensed spectrum, some unlicensed spectrums are further specified, so that without a need of paying and applying for, anyone can use a spectrum of this type to perform communication. The spectrum of this type is mainly distributed near 2.4 GHz or 5 GHz. Currently, a microwave oven, a medical device, and a wireless local area network in the industry all work by using the spectrum of this type.
As data volumes of wireless communication of users grow, frequency resources purchased by the operator are under increasing strain. Because a spectrum resource is non-renewable, increasingly high resource use fees drive the operator to develop the unlicensed spectrum. That is, some cells are established by using the unlicensed spectrum, sending is performed by using relatively low power, and a part of wireless communication traffic is offloaded at relatively low costs, so as to achieve an objective of improving a communication rate of the user and improving user experience. Because of openness of the unlicensed spectrum, an unlicensed-spectrum cell is deployed differently from a licensed-spectrum cell. A base station needs to ceaselessly detect whether someone is using the unlicensed spectrum. If someone is using the unlicensed spectrum, a signal cannot be sent by using the unlicensed spectrum. Therefore, the unlicensed-spectrum cell cannot be continuous in tell's of time. That is, a time of occupying the unlicensed spectrum is extremely limited, but a relatively long time is needed to achieve downlink synchronization in an existing synchronization method. Therefore, in the limited time of occupying the unlicensed spectrum, a time actually used for data transmission is compressed, which reduces utilization of the unlicensed spectrum.